Friction clutch arrangement

ABSTRACT

A friction clutch arrangement for motor vehicles to control a clutch friction disk rotationally mounted on a transmission shaft which can be locked between two components has a clutch actuating device includes a slave device adjacent to the clutch, a master device remote from the clutch and a force transmission system between the two devices. The slave device has an annular housing having an axial extension with a radially outer part and a radially inner part which are arranged concentrically to form an annular space containing a movable piston which is engagable with the clutch friction disk. A piston sensor sealed in the radially outer part of the axial housing extension makes it possible to determine the position of the piston in the slave cylinder independently of external influences such as vibrations or dirt or of the influence of neighboring components.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/EP/07187 filed Dec. 19, 1997.

BACKGROUND OF THE INVENTION

This invention relates to friction clutch arrangements for motor vehicles.

British Patent No. 2,256,907 discloses a friction clutch arrangement having a clutch disk which can be locked between two components and which is rotationally fixed to a transmission shaft, a clutch spring for locking the clutch disk between the two components, a clutch actuating device with a slave device situated near the clutch, a master device remote from the clutch, and a force transmission system between said two devices. In this arrangement, the slave device located near the clutch includes a fixed housing concentrically surrounding the transmission shaft and having two radially spaced axial extensions which are arranged concentrically to form an annular space and a piston that is movable within the annular space and is frictionally connected to the clutch disk. The slave device includes a piston motion sensor with a variable resistance, a variable capacitance, or a variable induction. The function of the piston motion sensor of this arrangement can be impaired by contaminants such as constituents of the clutch lining or oil escaping from the engine compartment.

U.S. Pat. No. 4,705,151 also discloses a friction clutch arrangement in which a slave cylinder of a slave device has a piston sensor with a variable inductance, capacitance, resistance or induction. The piston sensor described therein is only provided to indicate an extreme position of the piston, however. Detection of the piston position over the entire displacement path is not possible with such a sensor.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a friction clutch arrangement which overcomes disadvantages of the prior art.

Another object of the invention is to provide a friction clutch arrangement in which the position of the piston in a fixed slave cylinder of a slave device can be sensed without interference from external conditions as contamination, vibration and the influence of adjacent components.

These and other objects of the invention are attained by providing a friction clutch arrangement having a piston sensor mounted in a radially outer portion of a housing in which the piston is movable. As a result, the piston sensor is protected from contaminants such as parts of the clutch lining or oil vapors escaping from the engine compartment.

Preferably, the radially outer portion of the housing is nonmagnetic in the region between the piston sensor and the piston, thus preventing shielding of the electric field of the piston sensor.

Any adjacent ferromagnetic components, such as a transmission shaft or other parts, have no influence on the measurement signal because the magnetic permeability μ_(r) of ferromagnetic components is significantly greater than 1. The eddy currents induced in the ferromagnetic components are therefore negligible in comparison with the eddy currents in the nonferromagnetic components, which have a magnetic permeability μ_(r) of approximately 1. Also, vibrations and contaminants have no effect on the measurement result, as experiment has shown.

It is particularly advantageous for the nonferromagnetic, electrically conductive metal to be aluminum. Aluminum has the advantage that it has a particularly low electrical resistance and is relatively light. Because of its good electrical conductivity, an especially good measurement signal is achieved. The low weight of aluminum is advantageous in that it reduces the mass which must be moved during the motion of clutch disengagement.

Preferably the piston sensor is a sensor coil cable that is mounted in bores in the exposed end of the radially outer extension of the housing, which are then potted so as to be pressure sealed. In this way, the piston sensor is reliably protected against contaminants.

It is advantageous for the piston sensor to be elongated and to extend essentially parallel to the displacement path of the piston. This assures that any displacement of the piston, and thus any axial disengagement motion of the friction clutch part, produces a measurement signal, while other movements such as vibrations, for example, have very little effect on the measurement signal.

It is also preferable that the piston is made of or contains or is joined to a nonferromagnetic, electrically conductive metal. The movement of the piston induces an eddy current in the nonferromagnetic, electrically conductive metal, causing the inductance of the piston sensor to change. The piston sensor is part of a resonant circuit that also includes a capacitance and an inductance. Changing the inductance of the piston sensor changes the AC voltage present at the coil, and thus the resonant frequency of the resonant circuit. The change in resonant frequency then provides a measurement signal for displacement of the piston. With this sensor arrangement, only the piston sensor need by connected to an electrical cable; the piston itself need not be connected to an electrical cable. It is thus not necessary to connect any moving part to a cable.

BRIEF DESCRIPTION OF THE DRAWING

Further objects and advantages of the invention will be apparent from a reading of the following description in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a fluid-actuated friction clutch arrangement; and

FIG. 2 is a sectional view illustrating a representative embodiment of a slave device situated near the friction clutch of FIG. 1 which includes a piston sensor in a radially outer part of the piston housing.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 shows the basic structure of a typical fluid-actuated friction clutch arrangement. In this arrangement a clutch pedal 7 acts directly upon a master cylinder 6, which is connected by a force transmission arrangement consisting of a pressure line 5 leading to a slave device 2 mounted on a friction clutch 1. The slave device 2, which is affixed to the housing of the friction clutch 1 has an annular slave cylinder formed by a housing 8 and a guide bushing 9, along with a pressure chamber 13 and a movable piston 11 that closes the pressure chamber 13 and is movable in the slave cylinder.

Application of fluid pressure through the pressure line 5 to the slave device 2 makes it possible to open or close the friction clutch 1 and thus to control the transmission of force from an engine crankshaft 4 to a transmission shaft 3 of a vehicle.

A representative slave device 2 is shown in cross-section in FIG. 2. In this illustration, the slave device 2 is shown in the engaged condition.

The slave device 2, which is arranged concentrically in relation to the transmission shaft 3, consists of the annular slave cylinder formed by the housing 8 and the guide bushing 9, and a disengaging unit 18 that can be moved with respect to the slave cylinder. A bore 17 in the housing 8 connects the fluid pressure line 5 at one end and with the pressure chamber 13 at the other end.

The disengaging unit 18 includes a piston 11 and a release bearing 12 which includes a nonrotating bearing part 12a in axial contact with the piston 11 and a rotating bearing part 12b which is frictionally connected to a disengagable clutch part (not shown).

The pressure chamber 13 formed by the guide bushing 9 and an axial extension 16 of the housing 8 is closed by the movable piston 11. It will be understood that the guide bushing 9 and the housing 8 can be combined in one piece.

When the clutch pedal 7 is depressed, pressure is produced in the master cylinder 6 and in the pressure line 5 and the bore 17 in the pressure chamber 13. As a result, the piston 11 is moved into the disengaged position. This movement of the piston 11 is transmitted by the bearing part 12a, the bearing body 12c and the bearing part 12b to the disengagable friction clutch part (not shown) causing the crankshaft 4 to be disconnected from the transmission shaft 3.

Mounted in the housing 8 adjacent to the piston 11 is a piston sensor 10 that extends along essentially the entire displacement path of the piston.

The piston sensor 10 includes a sensor coil cable to which an AC voltage is applied. As a result of the AC voltage, the piston sensor 10 is surrounded by an alternating electric field which is affected by the presence and location of the piston, thereby providing a measurement signal. The cable 14 connects the piston sensor 10 to an electrical device for analyzing the measurement signal which is located outside of the slave device 2. The cable 14 can, of course, contain a number of conductors.

Provided in the housing 8 are bores extending in the direction of the displacement path of the piston 11 in which the sensor coil cable is arranged. On the axial faces of the housing 8, openings 15 are provided through which the sensor coil cable is inserted. Thereafter, the openings are potted so as to be pressure sealed. In this way, the piston sensor is reliably protected against contaminants, corrosion and the like.

The piston 11 is made of a nonferromagnetic, electrically conductive metal, or at least contains a nonferromagnetic, electrically conductive component or is securely affixed to such a component. The movement of the piston 11 in a direction perpendicular to the alternating field induces an eddy current in the piston 11. Nonferromagnetic, electrically conductive metals have a magnetic permeability μ_(r) of approximately 1. In contrast, ferromagnetic conductive metals have a magnetic permeability μ_(r) that is significantly greater than 1. The eddy currents induced in ferromagnetic components, such as in the transmission shaft 3, can therefore be ignored in comparison with the eddy currents induced in the piston 11. Detailed background information on the effects of magnetic permeability on induced eddy currents can be found in such publications as the VDI Fortschrittberichte [VDI Progress Reports], Series 8; Meβ-, Steuerung- und Regelungstechnik [Measurement and Control Technology], No. 557; Trennung der Einfluβgroβen von Wirbelstromsensoren durch Signalverarbeitung mit Hilfe von Felduntersuchung und Modellierung [Separating the variables that influence eddy current sensors by signal processing, using field study and modeling], by M. Sc. Yungiang Wang, Kassel, Germany.

The induced eddy current in the piston 11 opposes the alternating electric field in accordance with Lenz's Law, thereby changing the inductance of the travel sensor 10 and thus the AC current present in it.

The change in AC voltage, or if the travel sensor is located in an electric resonant circuit with a capacitance, the change in resonant frequency of the resonant circuit, then serves as the measurement signal.

Although the invention has been described herein with reference to specific embodiments, many modifications and variations therein will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention. 

We claim:
 1. A friction clutch arrangement for motor vehicles by which a clutch can be actuated comprising a friction clutch, a clutch actuating device having a slave device located adjacent to the friction clutch, a master control device at a location spaced from the friction clutch, and a force transmission system extending between the master control device and the slave device, wherein the slave device comprises an annular housing having an axial extension with radially outer and inner parts which are arranged concentrically to provide an annular space between them, a piston movable within the annular space and connected to the friction clutch and a piston sensor mounted in the radially outer part of the axial extension, wherein the piston sensor comprises a sensor coil cable and an axial bore in the radially outer part of the axial extension in which the sensor coil is mounted and an annular free space provided on an end face of the radially outer part of the axial extension within which the sensor coil is wound.
 2. A friction clutch arrangement according to claim 1 wherein a portion of the radially outer part of the axial extension between the piston sensor and the piston is nonmagnetic.
 3. A friction clutch arrangement according to claim 1 wherein the piston sensor is elongated in a direction essentially parallel to the displacement path of the piston.
 4. A friction clutch arrangement according to claim 1 wherein the piston comprises a nonferromagnetic, electrically conductive metal.
 5. A friction clutch arrangement according to claim 4 wherein the metal is aluminum.
 6. A friction clutch arrangement according to claim 1 wherein the radially outer and radially inner parts of the axial extension are separate components.
 7. A friction clutch arrangement according to claim 1 wherein the piston sensor comprises a variable inductance.
 8. A friction clutch arrangement according to claim 1 wherein the bore and free space are potted so as to be pressure sealed after the winding has been completed. 